This invention relates to wear parts and a method for the repair of wear parts and more particularly to the repair of the wear parts of wood chippers.
Conventional wood chippers for reducing slabs, logs or even entire trees to wood chips generally employ a rotatably mounted cutting disc. The cutting disc is usually formed from a mild steel and may be approximately five inches thick. A plurality of cutting blades or knives are mounted adjacent apertures on the cutting disc to be rotated therewith. The rotating disc is mounted on a base structure and typically includes a wear plate mounted to the face of the disc. The cutting knives which may be from 14 to 28 inches in length are secured to the aperture sides of the disc by a knife holder, counter knife and knife clamp, all of which are bolted directly or indirectly to the rotating disc. A housing surrounds the disc and includes an opening through which the wood to be chipped may be passed. The opening is covered by a spout which terminates in a feed plate adjacent an anvil. During rotation of the disc, the cutting knives are passed adjacent the face of the anvil and at such an angle so as to draw in the raw material to produce wood chips which are expelled through an exhaust chute usually at the periphery of the housing. The wood chips are generally employed in the paper mill industry.
The anvil, the feed plate, the wear plate mounted on the rotating disc, the knife clamp, and the knife holder are all subjected to wear due to the extreme frictional and impact forces incurred during operation. Since close tolerances are involved with respect to the clearance between the stationary and the rotating parts and due to the relatively high rpm at which the massive steel disc rotates, these wear parts must be replaced quite frequently to maintain efficient operation of the chipper.
Due to the relatively high cost of each of these individual wear parts, attempts have been made to repair the worn parts for reuse. Since these wear parts are subject to extreme stresses, are machined for close tolerances with respect to the individual parts to achieve the proper clearances between the rotating and nonrotating parts in order to obtain satisfactory chipping, and the fact that the rotating disc must be kept in balance, prior attempts to repair them have been entirely unsatisfactory.
For example, it has been proposed to manufacture the wear parts with a replaceable mechanically attached wear plate using bolts or other fasteners. Due to the shock loads encountered by the wear parts during use, the bolts generally fail or loosen. This is an extremely dangerous situation since the metal parts may be flung from the cutter or chipper at a high rate of speed resulting in possible injury to operating personnel and destruction of the chipper. Further, the use of mechanical members to attach inserts in the repair of the wear elements has resulted in an imbalancing of the rotating disc, thereby subjecting all parts of the chipper to vibration and subsequent cyclic fatigue.
It has also been proposed to resurface the worn areas by applying a filler material directly to the metal parts as with a metal arc welding process. The weldment was then ground to size. The generally high temperatures involved in an arc welding process have resulted in warpage and twisting of the wear parts. As a result, the attempted repair is totally unsatisfactory in that the necessary tolerances with respect to the wear parts and the necessary clearances in the chipper have not been obtainable.
Finally, attempts have been made to machine a recess in the wear part at the wear area and fill the recess with a mosaic of tungsten carbide inserts. The inserts are brazed at a temperature of 1600.degree. F to the wear parts. This has been an unsatisfactory method of repair with respect to the wear parts of the wood chipper, since the differences in the thermal expansion of the tungsten carbide insert and the base metal of the wear part have resulted in warpage and twisting of the part upon the attempt to repair it. In both of the latter efforts, the wood chippers either exploded or would not operate properly.
It is, therefore, readily apparent that all prior attempts to repair or resurface the wear parts found in wood chippers have been totally unsuccessful due to the exacting manufacturing tolerances involved, the close clearances between the rotating and nonrotating parts, the necessity of maintaining the cutting disc in balance and the fact that all prior methods attempted have resulted in twisting or warpage of the wear parts.